JP 2008-134388 A (corresponding to U.S. Pat. No. 7,733,495 B2) discloses a Fabry-Perot interferometer. The Fabry-Perot interferometer includes a pair of mirrors. Each mirror includes a pair of high-refractive layers each of which having a high refractive index and a low-refractive layer having a low refractive index. The pair of high-refractive layers is provided by semiconducting films made of silicon, germanium or the like. The low-refractive layer, which actually is a space layer, is selectively arranged between the pair of high-refractive layers. The pair of mirrors arranged facing each other via an air gap. Each mirror includes a bridge part that crosses the air gap. One of the bridge parts of the mirrors provides a membrane, which is movable. The bridge part includes a transmission portion in which the low-refractive layer is sandwiched by the pair of high-refractive layers and a periphery portion arranged around the transmission portion. The transmission portion at least includes one mirror element in which the low-refractive layer is sandwiched by the pair of high-refractive layers. The pair of transmission portions, respectively, included in the pair of bridge parts are arranged facing each other.
In the above Fabry-Perot interferometer, the mirror includes optical multiple layers including the space layer. With this configuration, a wide high-reflectance band is provided and, accordingly, a wide spectroscopy band is provided. However, a mechanical strength of each mirror having the space layer is low. Thus, a warpage may occur on the high-refractive layer arranged on the space layer. In order to secure the mechanical strength, a ratio of the space layer to the transmission portion may be reduced. That is, a width of the mirror element may be reduced.
Absorption wavelengths of normal gas and normal liquid, such as gasoline, water, alcohol, for example, ethanol, acetic acid, carbon dioxide, carbon monoxide, nitrogen oxide (NOx), sulfur dioxide are within a range of 2 micrometers (μm) to 10 μm, which is approximately equal to a mid-wavelength infrared range. Thus, the above-described Fabry-Perot interferometer may be used in an infrared light detector or may configure an infrared light absorption sensor together with an infrared light source. The infrared light detector and the infrared light absorption sensor may be used to detect compositions and concentration of a gas or a liquid.
However, when the width of the mirror element is reduced in order to improve the mechanical intensity, the mirror functions as a slit within the mid-wavelength infrared range and a diffraction occurs to a transmission light passing through the mirror. When the diffraction occurs, not only a rectilinear propagation light but also a diffraction light, which is slanted by the diffraction, resonate by the mirrors. When passing through the gap, an optical path length of the rectilinear propagation light is different from an optical path length of the diffraction light. Thus, a full width at half maximum (FWHM) of the transmission light, which passes through the Fabry-Perot interferometer, is increased. That is, a resolution of the infrared light absorption sensor to differentiate compositions is reduced. This conclusion is found by inventors of the present disclosure.